On Sun, 11 Sep 2005 20:56:34 GMT,
wrote:

It looks like the filter is designed for 50 ohms in and out. The transformer
inductance is also a part of the filter. It's value should be about the same as
L1 so to use it at 50k to 200K, you'll need to rewind for a lot more turns on
the primary and both secondaries if you keep the same core material.

Scaling it all and then optimizing for Coil Q of 40 and regular cap and coil
values, I get new values like:

L1 680nH - 100uH
C1 470pF - .022uF
C21 180pF - .027uF
L2 1.8uH - 100uH
C22 470pF - .022uF
T1 680nH (13:6:6 Turns) - 100uH (1911:882:882 Turns)

I strongly recommend changing T1 to a much higher mU core material.

This should result in -3dB points at 50 and 200 kHz, flat pass-band and -30dB at
20 and 500 kHz.



OK, thank you very much.

I checked the amidon website, higher mu toroids are available but
looks like they're a little more expensive.

If I want to simulate the filter myself, can I just ignore the series
10 ohm resistors in each side of the transformer secondary? Do the 5
ohm ac series resistance of the switch and the .1 uf capacitor on the
far side of the switch enter into the calculations at all????

I was thinking all those R's and C's get reflected back through the
transformer and should impact the filter values.

If I can ignore the R's and C's on the secondary side of the
transformer, can I simply pretend the primary is a coil with a 50 ohm
shunt (to represent the output impedance)??

When the series 10 ohm resistor, the 5 ohm series ac resistance of the
switch and the .01 cap to ground on the output side of the switch is
factored in, does the output impedance still look like 50 ohms?

Good luck finding a 50 Ohm antenna at these frequncies. An active antenna may be
the way to go, otherwise you'll need many acres for all the wire.



Yep, we can do it. Got 40 acres. But, wasn't planning on using that
many acres:: My plan was to use a single turn loop, 10 to 15 feet on
each side and step up the impedance with a toroid at the antenna. The
wire is 200 A service entrance insulated aluminum, so it should have a
decent Q if I use some polystyrene caps to resonate it.



Regards,

T

On Sun, 11 Sep 2005 12:09:16 -0400, TRABEM wrote:

I am trying to scale an existing front end receiver (butterworth
bandpass) filter to a different frequency range. Unfortunately, it has
a transformer in the original design, so I'm stuck. I also don't know
how to handle the load presented by the active front end component
other than it's probably not significantly reactive.

The existing filter is for a 7 Mhz receiver, I'd like to have a
similar filter design for 50 to 200 Khz.

The filter components and transformer winding details are in the
document at:

http://www.amqrp.org/kits/softrock40...0Assy%20v1.pdf

The input chip is an FST3126, spec sheet is at:

http://www.fairchildsemi.com/ds/FS/FST3126.pdf

The spec for the T30-2 transformer core is at:

http://partsandkits.com/T30-2.htm

I have aade filter design software, but it isn't allowing me to plug
in the transformer into the design page of the software....so, I need
to know it's equivalent circuit I think.

The transformer winding details are on page 4 of the document and the
schematic of the front end is page 9. Ultimately, I need new values
for L1, L2, C20, C21 and C22.

If someone can give me a reasonable guess as to the equivalent circuit
of the transformer and the IC (U5), I can do the rest of the job
myself using the aade filter software.

Thanks,

T